1. Field of the Invention
The present invention relates to an electrically driven closure apparatus for architectures, such as an electrically-driven shutter of a building.
2. Description of the Related Art
Generally, a closure apparatus having an electric motor to drive a closure member for opening and closing, for example, an entrance of a building has a risk of being jammed with an obstacle when being operated. Therefore, the apparatus preferably has an obstacle detector and an automatic stop controller that automatically stops the closure member when the obstacle detector detects the obstacle. Two types of the obstacle detector are known: one is a direct detecting type that detects an obstacle by a detecting sensor such as a seat plate switch provided on the closure member; the other is an indirect detecting type that detects an obstacle indirectly by detecting load variation or torque variation of the electric motor that occurs when the closure member is blocked by the obstacle. The indirect type detector has an advantage in that the closure member is not required to have an obstacle sensor and that the obstacle detector can also serve as a limit detector. Conventionally, the indirect type obstacle detector may detect the load variation in accordance with a variation of a rotation speed or an electric current value (or a voltage value). However, a general electric closure apparatus utilizes an electric motor working within a range where the variation of the rotation speed due to the load variation is small, so as to obtain a stable opening and closing speed. Therefore, there is a problem that the variation of the motor speed is small when the closure member is resisted by an obstacle. Another problem experienced with the current detecting type is that the electric current can also be varied by a disturbance other than the load variation. Thus, the above-mentioned detectors have difficulty in detecting an obstacle with high accuracy and good compatibility between detection sensitivity and operation stability.
In order to solve the above-described problem, a mechanism has been proposed which can perform stable opening and closing operation while detecting an obstacle accurately. In this mechanism, an electric motor or a driving device serves as a displaceable member that changes its position in accordance with the load variation. The displaceable member is supported by a load detecting spring (a neutral position keeping spring) that keeps, under a predetermined load, the displaceable member at a neutral position with respect to a fixed member that is fixed to a frame. A displacement sensor detects change in the position of the displaceable member that moves against the load detecting spring.
During the closing operation, it is necessary that the detection of obstacle be performed with high accuracy, in order that the electric motor is stopped without delay upon detection of any obstacle. In contrast, during the opening operation, the detection sensitivity is preferably set to a low level, in order to ensure smooth and stable movement of the closure member in the opening direction.
However, the conventional load detecting spring mentioned above employs a single substantially a U-shape spring member so that an equal spring constant is applied both in the closing and opening operations. This means that the levels of the detection sensitivity during the closing and opening operations are the same. Consequently, it has been necessary that the spring constant of the detection spring be set to a level which is a compromise between the high sensitivity required during closing and the low sensitivity required during opening. This is the problem to be solved by the present invention.